1. Field of the Invention
The present invention relates to a novel transparent film usable as an optically-compensatory film in liquid-crystal devices and as a protective film in polarizing plates, and also to a polarizing plate and a liquid-crystal display device comprising the transparent film.
2. Related Art
A transmissive-type liquid-crystal display device has a liquid-crystal cell and a pair of polarizing elements each of which is disposed either of sides of the cell. In general, the polarizing elements are so disposed that their transmission axes are perpendicular to each other, as so-called perpendicular polarizing elements. The perpendicular polarizing elements could correctly function as they are, for the incident light going through them in the normal direction to their surfaces, but could not function as perpendicular polarizing elements for the incident light going through them in the oblique direction relative to the normal direction, since crossing angle of the transmission axes shifts from a right angle. This causes light leakage in an oblique direction in a transmissive-type liquid-crystal display device, and is one of the factors worsening the viewing angle characteristics such as decreased contrast and color shift generating depending on the viewing angle.
In general, a polarizing element formed of a polyvinyl alcohol film or the like is not used in a liquid-crystal display device as a single member by itself, but, it is used in a liquid-crystal display device as a polarizing plate that is produced by sticking a protective film to both surfaces of the polarizing element to protect it. Some methods have been tried for broadening the viewing angle range for the device, by imparting predetermined optical characteristics to the protective film. Examples of a wide viewing angle polarizing plate are disclosed in Japanese Laid-open Patent Application No. 2001-350022. The publication discloses a wide viewing angle polarizing plate comprises a polarizing plate, with a biaxial optical retardation plate superposed thereon, which is an optical retardation plate has in-plate phase difference of 250-300 nm retardation in plane and 0.1-0.4 Nz; and a wide viewing angle polarizing plate comprises a polarizing plate, with a biaxial optical retardation plate superposed thereon, which is an optical retardation plate having in-plane phase difference of 250-300 nm retardation in plane and 0.6-1.1 NZ. According to the Example section of the publication, a transmissive-LCD was improved in terms of the viewing angle by employing these two wide viewing angle polarizing plates disposed at either of the sides of the cell respectively whose transmission axes are perpendicular to each other.
According to the viewing angle-broadening technology for polarizing plates described in the publication, the incident light must be made to run twice through a biaxial retardation plate having a predetermined birefringence. Accordingly, there should be some design limitation of a liquid-crystal display device, or there should require a complicated process of laminating two retardation plates having predetermined optical characteristics while the positional relation of their optical axes are accurately controlled, and therefore the technology is unfavorable in terms of producibility.
On the other hand, there has been tried to compensate the viewing angle dependency of a polarizing element by making the incident light thereto pass through only one retardation plate. However, this causes a problem of wavelength dependency. This may be because the retardation of a protective film of a polarizing element, which is generally selected from triacetyl acetate films, shows wavelength dependency. Even though the optical characteristics of a protective film are so controlled that the incident light at a wavelength of 550 nm (G), which is the center of a visible light wavelength range of from 400 to 700 nm, may be converted into an extinction point by passing through the protective film, the other incident light at 450 nm (B) and at 650 nm (R) may be converted into a polarized state shifted from the extinction point, and, as a result, the problem of color shift and decreased contrast generating depending on the viewing angle could not be still solved.